A subject of the invention is tris-hydroxyaryl compounds, preferably 1,1,1-tris(4-hydroxy-phenyl)ethane (THPE), with a content of metal ions, in particular sodium ions, of less than 5 ppm, preferably less than 1 ppm. A further subject of the invention is a process for the production of tris-hydroxyaryl compounds, preferably 1,1,1-tris(4-hydroxyphenyl)ethane (THPE), with a content of metal ions, in particular sodium ions, of less than 5 ppm, preferably less than 1 ppm, by purification of the aromatic hydroxy compounds on acid cation exchange resins.
Tris-hydroxyaryl compounds, in particular THPE, are suitable for the production of branched polycarbonates both by the known phase boundary process and also by the transesterification process with organic carbonates in the melt, so-called polycarbonate melt processes. The use of THPE for the production of branched melt polycarbonate is for example described in EP 1 458 786 A1. In contrast to the phase boundary process, the polycarbonate melt process sets higher requirements for the purity of the starting materials, since undesired by-products cannot be eliminated by a phase separation, but, in particular when they are heavy or non-volatile, such as for example metal salts, remain in the polymer melt and thus lead to undesired side-reactions. this also applies for tris-hydroxyaryl compounds which are used as branching agents in the production of branched melt polycarbonate and are commercially available products. Even traces of metal salts in the lower ppm range, which can be contained as by-products or impurities in such branching agents, have adverse effects on the catalysis of the melt polycarbonate process and lead, as is generally known, to a melt polycarbonate with a markedly increased content of rearrangement products, which for example arise through the so-called Fries rearrangement (A).
with X=isopropylidene residue.
The nature and quantity of these defective structures is dependent on various process parameters such as for example temperature, residence time and also above all the nature and quantity of the catalyst used. It is further known that alkali and alkaline earth metal compounds favour the formation of defective structures (e.g. see EP 1 369 446 A1 and EP 1 500 671 A1). Such defective structures are undesired in the melt polycarbonate, since they adversely affect the natural colour of the product and the polymer melt flowability.
Since some of the commercially available tris-hydroxyaryl compounds, such as for example THPE, contain residual contents of metal ions such as for example sodium ions of more than 5 ppm, there was thus a need for tris-hydroxyaryl compounds which are largely free from metallic by-products and an industrially usable process for the purification of tris-hydroxyaryl compounds.
The production of tris-hydroxyaryl compounds, in particular the production of THPE, is known from the literature and for example described in EP 782 978 A1, EP 765 852 A1, EP 930 289 A1, EP 847 975 A1 or EP 441 648 A1. Thus EP 765 852 A1 discloses the synthesis of THPE with the use of ion exchangers and cocatalysts; EP 930 289 A1 discloses the same synthesis with the use of mineral acids. The synthesis of the tris-hydroxyaryl compounds is effected with a large excess of one of the educts, in particular the hydroxyaryl, in order to shift the equilibrium in the direction of the tris-hydroxyaryl compounds. Although the end products in particular according to EP 765 852 A1 should be free from metal ions, residual contents of sodium over 5 ppm are found in the commercial products. Possible causes of these metallic impurities are subsequent treatments of the crude products of THPE with sodium borohydride, as for example described in EP 782 978 A1, or the use of sodium-containing cocatalysts, such as for example sodium mercaptopropanesulphonate, as disclosed in EP 847 975 A1. The addition of sodium-containing stabilisers such as for example sodium dithionite, e.g. disclosed in EP 441 648 A1, can also cause increased sodium contents in commercial THPE. A reduction in the content of other by-products, such as for example sulphur compounds, in the THPE by a subsequent treatment of the THPE with sodium hydroxide solution, see EP 646 613 A1 on this, can also cause increased sodium contents in the THPE end product.
None of the above references describes a process, which leads to a tris-hydroxyaryl end product which is largely free from metallic impurities and is suitable for the production of branched polycarbonate by the melt process wherein the formation of undesired defective structures (Fries structures, xanthone structures) is as far as possible avoided.
There was thus a need for a process with high yield, whereby commercial tris-hydroxyaryl compounds are as far as possible freed of contained metal cations.
The purpose of the invention was thus to provide tris-hydroxyaryl compounds, in particular THPE, with residual metal ion contents of less than 10 ppm, preferably less than 5 ppm in particular from 0.01 to 5 ppm, which overcome the disadvantages of the aforesaid production processes. A further purpose of the invention was to attain residual contents of sodium ions below 5 ppm, preferably less than 1 ppm, in particular from 0.005 to 1 ppm.
Surprisingly, this purpose according to the invention was solved in that commercially available, metal-containing tris-hydroxyaryl compounds are treated under precisely defined conditions with acid cation exchangers, without appreciable observation of the cleavage of the tris-hydroxyaryl compounds back to the starting materials to be expected during this. In particular the expected acid-catalysed removal of phenol and formation of by-products could not be appreciably seen. The tris-hydroxyaryl compounds thus purified are obtained in yields of more than 95% with this process according to the invention and have metal ion contents of less than 5 ppm, preferably less than 1 ppm. On use as branching agents in the melt polycarbonate process, they cause no undesired side-reactions or colour changes.